True if one is adding paint colors but, but here we are talking about adding light and that is rather different. The primaries that matter for film are red, blue, and green. So, with the light impinging upon film, the green light is not the product of adding two other colors.
Originally Posted by JPD
Primaries are red, blue, and green
Adding primaries gets you the secondaries, cyan, magenta and yellow.
Red and blue = magenta,
red and green = yellow,
green and blue = cyan.
Red and blue and green (in the right proportions) = white
In a darkened room if you put the bright spot a strongly gelled red spotlight on a white wall, and then direct the spot from a strongly gelled green spot on top of it, the light you get is yellow. Not intuitive at all.
For paint - things are more inutitive:
Primaries are red, yellow, and blue.
Adding primaries gets you the secondaries.
Red and blue = purple,
red and yellow = orange,
blue and yellow = green.
Red and blue and green (in the right proportions) = black
So, flipping your sentence around - yellow has some green in it.
So... For film and filters (and I am talking about strong filters):
Primaries - the blue filters pass only blue,
red filters pass only red,
green filters pass only green.
Secondaries - magenta filters pass red and blue,
yellow filters pass green and red,
cyan filters pass green and blue.
Among other consequences, all that means that to mimic a plain emulsion with a pan film and filters, use a blue filter. To mimic an ortho emulsion, use a cyan filter which passes blue and green.